The present invention relates to an apparatus and method for repairing a weld joint between pipes that have been welded together. In particular, but not exclusively, the invention relates to repairing welds between pipe sections when laying pipelines underwater at sea.
When laying a pipeline at sea from a pipe-laying vessel, one end of the pipeline (sometimes referred to as the “pipe string”) is held by the vessel and a pipe section is welded to the end of the pipeline. The pipeline and newly welded pipe section are then fed out from the vessel, so that the process can be repeated. The weld joint must be of high quality in view of the high tensile loads to which the pipeline is subjected during the laying process.
A known technique for adding a new pipe section to the end of a pipeline includes beveling the ends of the pipes to be joined, arranged the pipes end to end thereby defining a circumferential groove to be welded, and then welding the pipes together by means of one or more externally mounted semi-automated GMAW welding heads which deposit weld material in the groove. The quality of the weld may then be checked by means of non-destructive testing (NDT), such as ultrasonic testing and a visual inspection performed with the aid of cameras. If a weld defect, for example a crack or lack of fusion, is detected the defect will be repaired if feasible. A known method of effecting such a repair includes removing weld material by means of “arc air gauging”, followed by manually grinding the resulting excavation, and then manually filling the excavation with weld material by means of a shielded metal arc welding (SMAW) method. The quality of the repair is inspected manually by means of an ultrasonic probe. The repair process described above typically takes an hour to complete. During the repair operation, other activities on board the vessel are suspended until the repair is completed. Production delays due to such repairs are extremely expensive. If a repair is not feasible, the pipeline is retracted, cut and the process of joining a new pipe section to the end of the pipeline is started again. Cutting the pipeline in this fashion is rare as it severely delays production, and as such, performing a repair is the preferred option.
The above described method of removing weld material includes remelting the material and blasting it out of the joint by means of a high-pressure airflow, leading to spatter and an irregular excavation. Irregularities in the excavation can make re-welding the joint properly a difficult task.
The present invention seeks to mitigate the above-mentioned problems. Alternatively or additionally, the present invention seeks to provide an improved method of laying a pipeline, an improved weld repairing method, and/or an improved pipe-weld repairing apparatus.